A team of security researchers has uncovered a high-severity new attack that could make up to one third of all HTTPS web traffic vulnerable to compromise. The so-called DROWN attack (CVE-2016-0800) is a cross-protocol attack that exploits flaws in the SSLv2 protocol.

The attacker must passively observe around 1,000 TLS handshakes and initiate roughly 40,000 probe connections, performing computations offline to complete the attack. Running the computations on Amazon EC2 costs around $440.

The report indicates that 25% of the top one million domains, and 33% of all HTTPS sites, are vulnerable to the DROWN attack. The attack is focused on server systems, which typically manage the HTTPS process. The researchers add, “There is nothing practical that browsers or end-users can do on their own to protect against this attack.”

The team that discovered DROWN has gone to considerable trouble to make information available to users. A website that went live at the moment of public disclosure includes a testing tool to check whether your systems are vulnerable.

Users are encouraged to disable SSLv2 “… in all SSL/TLS servers if you haven't done so already.” Disabling SSLv2 ciphers without disabling the protocol is not sufficient, unless you have updated your systems with the patches for an earlier SSL problem (CVE 2015-3197), because an attack could force SSLv2 if it is present on the system.

The team also cautions not to share private keys among servers. According to the DROWN website, “Many companies reuse the same certificate and key on their web and email servers, for instance. In this case, if the email server supports SSLv2 and the web server does not, an attacker can take advantage of the email server to break TLS connections to the web server.”

See the technical paper for additional information on the DROWN attack.